The invention relates to aqueous dispersions of polyurethane resins free from urea groups and containing ionic groups and of use as binder dispersions for coating agents. The invention also relates to manufacture of the dispersions and coating agents containing them and use thereof.
The use of binders dilutable after neutralisation in water is very important in the lacquering industry. The literature also contains descriptions of polyurethane dispersions of general use as binders in the lacquering industry. Polyurethane dispersions, particularly in multi-layer lacquer coats, have good properties as binders in primers, base lacquers or covering lacquers. When used as base lacquers, the dispersions are preferably covered wet-on-wet with clear lacquers, after which the layers are simultaneously cross-linked by baking.
Various synthetic methods of producing water-dilutable polyurethane resins are known. One problem in synthesis is to obtain a high molecular weight. This is generally done by preparing pre-adducts which are subjected to a chain-lengthening reaction. According to EP-A-0 089 497, dispersions of water-soluble isocyanate prepolymers are prepared and are lengthenedby reacting with diamine chains in the aqueous phase. In DE-A 39 15 459, isocyanate-terminated pre-adducts are produced, and after neutralisation, are converted into the aqueous phase. As a result of the excess of water in the dispersion phase, a chain-lengthening reaction occurs among the dispersed particles. In DE-A-35 45 618, NCO-terminated prepolymers are prepared and converted into the aqueous phase. Water-dispersible polyols are then introduced into the aqueous phase and react with the NCO groups and increase the molecular weight.
It has been found, however, that the chain-lengthening reactions of NCO-terminated reaction products in the aqueous phase are difficult to reproduce, and the same consequently applies to the high molecular weights.
In DE-A-40 01 841, an aqueous dispersion of an amino group-containing prepolymer is prepared. The pre-polymer is reacted with a water-insoluble polyisocyanate in the aqueous phase, resulting in a longer-chain polyurethane polymer. The resulting binders have the disadvantage of an increased content of urea groups, i.e. a hard polymer constituent which impairs the properties of the binder. In DE-A-39 03 804, polyesters containing hydroxyl groups and carboxyl groups are reacted to obtain a polyurethane prepolymer containing OH groups. This prepolymer is reacted in the solvent phase with triisocyanates, resulting in partly cross-linked products, and is then neutralised with amines and converted into the aqueous phase. One disadvantage of the resulting binders is that they are sensitive to hydrolysis, so that the aqueous binder dispersion is difficult to store for a prolonged period.